Murine neocortical cell cultures were transiently deprived of both oxygen and glucose, producing widespread neuronal swelling in less than 60 min, followed by neuronal degeneration over the ensuing several hours, despite return to normal medium. Cultured glia (> 95% astrocytes) were irreversibly injured only by oxygen-glucose deprivation exposures exceeding 4–6 hr. Replacing either Na+ or Cl- with impermeant ions blocked acute neuronal swelling but did not prevent delayed neuronal degeneration. While neuronal swelling and death were increased by removing Ca2+ from the exposure medium, combined removal of extracellular Ca2+ together with Na+ or Cl- substitution blocked both acute and delayed injury. If acute swelling was limited by a hyperosmolar medium, then neuronal loss depended on extracellular [Ca2+]. Oxygen-glucose deprivation was associated with a large increase in extracellular glutamate concentration. Both early swelling and later neuronal degeneration were blocked by addition of NMDA receptor antagonists to the exposure medium but not by the AMPA/kainate receptor antagonist 6-cyano-7-dinitroquinoxaline-2,3-dione (CNQX), dihydropyridines nifedipine or nimodipine, or TTX. Oxygen- glucose deprivation induced substantial neuronal uptake of tracer 45Ca2+ from the exposure medium that was reduced by NMDA receptor antagonists and closely paralleled the degree of subsequent neuronal loss. These observations suggest the presence of two distinct components of hypoxic injury, each involving NMDA receptor activation and each capable of leading to neuronal death. Acute swelling is mediated by influx of Na+, Cl-, and water, and is enhanced by removal of extracellular Ca2+. Delayed neuronal degeneration depends on the presence of extracellular Ca2+ and correlates closely with cellular uptake of 45Ca2+.